The truck was found close to where it was stolen outside Mexico City, said the official, who asked not to be identified in line with policy.

The AP is quoting a Mexican source that the radioactive material was missing from the truck.

The truck was stolen on Monday while it was taking cobalt-60 from a hospital in the northern city of Tijuana to a radioactive waste-storage center, Mexican officials and a U.N. agency said earlier.

The truck was carrying a metal container of cobalt-60 headed to a nuclear waste facility in the state of Mexico, said Juan Eibenschutz, director general of the National Commission of Nuclear Safety and Safeguards. Though the container is heavily sealed in lead, designed to be difficult to break and to survive accidents intact, he said it contains an amount of radioactive material that could do serious damage if opened.

Direct exposure would result in death within a few minutes, he added.

“This is a radioactive source that is very strong,” Eibenschutz told The Associated Press on Wednesday, adding that it can be almost immediately fatal, depending on proximity. “The intensity is very big if it is broken.”

Eibenschutz didn’t know the exact weight, but that it was the largest amount stolen in recent memory, and the intensity of the material caused the alert. Local, state and federal authorities, including the military, are searching for the truck.

The material was used for obsolete radiation therapy equipment that is being replaced throughout Mexico’s public health system. It was coming from the general hospital in the northern border city of Tijuana, Eibenshutz said. The thieves most likely wanted the white 2007 Volkswagen cargo vehicle with a moveable platform and crane.

There are an average of a half-dozen reported thefts of radioactive material in Mexico each year, Eibenschutz said, and none have proven to be intentional, meaning the thieves were not after the material. He said in all cases so far, they were after the containers or vehicles.

Such unintentional thefts are not uncommon, said an official familiar with cases reported by International Atomic Energy Agency member states, who was not authorized to comment on the case. In some cases, radioactive sources have ended up being sold as scrap, causing serious harm to people who unknowingly come into contact with it.

In a famous case in the 1970s of stolen radioactive material in Mexico, one thief died and the other was injured when they opened the container, he said. The container was junked and sold to a foundry, where it contaminated some of the steel reinforcement bar that was made there. Eibenschutz said all foundries in Mexico now have equipment to detect radioactive material.